The results of XENON1T, the world's most sensitive and largest detector for “Weakly Interacting Massive Particles” (WIMPS), were presented today at the Italian Gran Sasso laboratory. WIMPs are the leading hypothesis for dark matter and are therefore searched for by multiple collaborations and using different approaches. Up to a billion WIMPs are predicted to traverse a square meter on earth per second but these particles have remained hard to detect. The XENON1T detector attempts to detect the elusive particles by the recoil they would introduce when they scatter off xenon atoms. “In 1000 kg of xenon we only expect a few of these interactions a year,” says Jan Conrad, leader of the XENON group in Stockholm. After one year of taking data with XENON 1T there remains no evidence for WIMPs.
XENON1T is operated by an international team of 165 researchers from twenty seven universities. The detector is cylindrical, about 1 meter tall and 1 meter wide and filled with liquid xenon which is cooled to -95 degrees to increase the interaction probability with WIMPs. The recoiling atoms give rise to short flashes of light and electrons that can be detected with sensitive photosensors. Unfortunately, an overwhelming background of other interactions can mimic WIMP scattering events, for example, the natural radioactivity that is present in all materials and cosmic rays. To mitigate this problem, special materials were chosen to build the detector and it was placed deep underground. The XENON1T detector was also designed to yield as much information as possible about the recoil that happened and advanced statistical methods have been developed to search for the small signal expected from an interaction with a dark matter particle. “We have not only taken more data, we worked hard to improve analysis. I am very satisfied that we achieved our goals,” says Knut Morå, PhD student at Stockholm University and the main researcher responsible for statistical analysis in XENON1T.
XENON1T will be upgraded to a larger detector next year. XENONnT, as it is called, will be four times larger with the aim to increase sensitivity another order of magnitude.
The group in Stockholm, after joining the collaboration in 2015, has a number of key responsibilities in the XENON collaboration. “ We cannot deny that we hoped for a discovery, but the next step, namely XENONnT will really be the crucial leap in sensitivity. A discovery should be around the corner,” says Jan Conrad.
Additional information:
The participation of the Stockholm group in the XENON experiment has been made possible by a grant from the Knut and Alice Wallenberg Foundation.
